The modifier-of-deaf waddler (mdfw) is a hearing susceptibility locus which confers progressive hearing loss to CBy-dfw2J/+ heterozygous. Neither homozygosity at the mdfw locus nor heterozygosity at the dfw2J locus alone are sufficient to cause the observed hearing impairment indicating a synergistic interaction between the two loci. We identified the dfw gene and showed that mutations in the plasma membrane Ca2+ ATPase 2 gene (Atp2b2) are the cause of hearing loss and imbalance in two mutant strains of deaf waddler. We are now focusing on identifying the second component (mdfw) of this genetic interaction. We expanded the mapping cross and constructed an integrated genetic, radiation hybrid and BAC based physical map of markers around mdfw. We also fine-mapped the waltzer (v) mouse mutation and showed that v and mdfw localize to the same genetic interval. In the waltzer locus, we identified a novel cadherin gene, called Cadherin-23 (Cdh23). We identified mutations in Cdh23 in alleles of waltzer that cause the deafness/waltzing syndrome. We showed that the phenotype is due to a severe stereocilia disorganization in auditory and vestibular hair cells. Our discovery also led to the identification of human CDH23; mutations in the human homologue were shown to cause the deaf/blind disease Usher Syndrome type 1D. To evaluate Cdh23 as candidate gene for mdfw, we determined the nucleotide sequence of Cdh23 in several inbred strains, including CAST/Ei, V/Le, CBA/CaJ and BUB/BnJ. Between the BALB/c and CAST/Ei-derived sequence we found eleven amino acid substitutions; however none of these changes correlated with the segregation of mdfw. Age-related hearing loss (AHL) in mice is a genetically complex trait and parallels pathology and etiology of sensorineural presbyacussis in humans. A major quantitative trait locus (QTL) for AHL has been mapped to mouse chromosome 10. We used a positional cloning strategy, based on segregation analyses in multiple crosses, to identify a synonymous single-nucleotide polymorphism (SNP) in Cdh23 associated with AHL and the deafness modifier mdfw (modifier-of-deafwaddler). The 753G>A SNP shows significant association with hearing thresholds in 56 inbred strains. The Cdh23753A allele causes in-frame skipping of exon 7, producing a protein with two truncated cadherin-repeats near the amino-terminus. Homozygosity at Cdh23753A significantly increases susceptibility for AHL, but is not the only cause of its phenotypic manifestation. In fact, predisposition to early-onset AHL conferred by Cdh23753A depends on the effects of several strain specific genetic factors, including the mitochondrial mutation mt-Tr9827ins8 (as in A/J), ahl2 (as in NOD/LtJ), and ahl3. A combination of either one of these "accelerating alleles" with Cdh23753A is sufficient to induce AHL. A further genetic factor is the null allele of the Ca2+ ATPase ATP2B2, which is an important regulator of intrastereocilia Ca2+ levels. Haploinsufficiency at Atp2b2 and homozygosity of Cdh23753A together, but neither alone, cause early-onset hearing loss in mdfw mice (Atp2b2+/dfw-2J mdfw/mdfw). The heterogeneity of secondary factors suggests indirect interactions with Cdh23753A. Altered adhesion or reduced stability of otocadherin may confer susceptibility to AHL. The genetic architecture of AHL and NIHL potentially provides a paradigm for predisposition to age-related and noise-induced hearing loss in human and defines a presbyacussis model to explore therapeutic avenues such as stem cell therapy. Our study in principle solves the problem of the genetics of age-related hearing loss for most of the common laboratory inbred strains of mice and identifies the deafness modifier mdfw. Polyclonal antisera were raised against Cdh23 to study protein levels and sub-cellular distribution in hair cells of AHL-positive and AHL-negative strains. Preliminary analyses indicate that cadherin 23 locaaalizes to the tip of growing stereocilia. Western-blot analyses show that cadherin 23 is highly expressed in the cochlea during the first two weeks of postnatal development, declines during the following two weeks and sustains a low level of expression during adult